local MAIN branch error: /examples/ads.amp.html could not be transformed by the postHTML pipeline

### Description

Just pulled main branched, executed `npm install` and command: `amp` after it.
And I am not seeing any of the ad adapters and I am getting this error in the console:

```
/examples/ads.amp.html could not be transformed by the postHTML pipeline.
ENOENT: no such file or directory, open 'C:\Users\damja\Documents\amphtml\examples\ads.amp.html'
```

### Reproduction Steps

- Git clone main branch of amphtml
- run: `npm install`
- run: `amp`
- go to http://localhost:8000/examples/ads.amp.html

### Relevant Logs

```shell
/examples/ads.amp.html could not be transformed by the postHTML pipeline.
ENOENT: no such file or directory, open 'C:\Users\damja\Documents\amphtml\examples\ads.amp.html'
```

### Browser(s) Affected

Chrome

### OS(s) Affected

Windows 10

### Device(s) Affected

_No response_

### AMP Version Affected

latest main branch 1.2.2022

Start by reproducing the Windows 10 setup with `npm install`, then run `amp` and request `examples/ads.amp.html`. Inspect why the postHTML pipeline looks for the missing file; done means the page transforms and loads without the ENOENT error, with the ad adapters visible.

Written by the indexing model from the issue text.